1. Field of the Invention
The present invention relates to a surface acoustic wave filter device. In particular, the present invention relates to a surface acoustic wave filter device including a ladder type surface acoustic wave filter unit.
2. Description of the Related Art
In the past, for example, as band pass filters or demultiplexers installed in RF (Radio Frequency) circuits in communication devices such as cellular phone devices, surface acoustic wave filter devices, each utilizing a surface acoustic wave, have been used. As an example of such a surface acoustic wave filter device, for example, in the following Japanese Unexamined Patent Application Publication No. 2000-196409, a surface acoustic wave filter device has been described that includes a ladder type surface acoustic wave filter unit. In Japanese Unexamined Patent Application Publication No. 2000-196409, it has been described that, in a plurality of surface acoustic wave resonators configuring a ladder type surface acoustic wave filter unit, a protective film is provided on the IDT electrode of each surface acoustic wave resonator and by causing the thickness of a protective film on an IDT electrode in a parallel arm surface acoustic wave resonator and the thickness of a protective film on an IDT electrode in a series arm surface acoustic wave resonator to be different from each other, a filter characteristics is caused to be steep and a frequency characteristic in a pass band is caused to be flat.
However, in the surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 2000-196409, there is a problem that a ripple occurs within the pass band.